1. Field of the Invention
The present invention relates generally to a liquid storage vessel with an unsecured top and specifically to an above ground gas tight liquid storage tank having a floating roof. More specifically the present invention is a secondary sealing arrangement for use in sealing the oil storage tanks that have been in use until the advent of a deformable seal.
2. Description of the Prior Art
U.S. Pat. No. 3,926,332 entitled Sealing Structure for a Liquid Storage Vessel Having A Floating Head issued to Chikashi Y. Okamoto on Dec. 16, 1975, teaches a sealing structure for use in a liquid storage vessel such as an oil storage tank having a floating roof. The sealing structure comprises a hollow resilient core of generally loop form, with a core having a substantially arcuate cross-section which is allowed to contract or expand in its cross-section when subjected to stress as the floating roof moves relative to the body of the storage vessel. This sealing structure is presently the best device for hermetically sealing the tank between the tank body and the floating roof without applying excessive pressure. The difficulty with this device is that it entails refitting all the storage vessels in present use with this new sealing structure. This refitting represents a substantial cost expenditure on the part of the petroleum industry and other industries which require the use of liquid storage vessels with floating roofs.
U.S. Pat. No. 3,106,309, entitled Secondary Seal for Floating Tank Roof, issued to William R. Nelson and Marshall U. Bagwell on Oct. 8, 1963, teaches a primary seal that includes a number of steel shoes, each of which is supported by and interconnected with the floating roof by a counterweight hanger structure. By appropriate balance of the counterweight the steel shoes of the primary seal are held in contact with the sidewall of the tank as the floating roof rises and falls in accordance with changes in the amount of the contents stored in the tank. The patent also teaches a secondary seal which is positioned with a slot therein for fitting over the break in the steel shoe for support thereby. The secondary seal is a flexible member, generally rubber or a synthetic resilient material, and is adapted to press against the sidewall of the tank.
U.S. Pat. No. 3,275,183, entitled Secondary Seal, issued to Kenneth D. Challenger on Sept. 27, 1968, teaches that in the prior art guide shoes were provided which were attached to the floating roof and which were resiliently biased against the sidewall of the tank. The guide shoes also included a flexible seal member extending from the guide shoe to the sidewall of the tank to prevent foreign material from falling through the annular space into the oil or oil products stored below the floating roof. To further insure against leakage a secondary seal of a resilient material, such as rubber, extending between the guide shoes and the sidewall of the tank was also used. The secondary seal had a groove for sealingly engaging the upper edges of the guide shoes and were forced into a sealingly engagement with the sidewall of the tank.
U.S. Pat. No. 2,314,805, entitled Floating Tank Roof Seal, issued to John H. Wiggins on Mar. 23, 1943, teaches a floating tank roof seal that includes an annular shoe carried by hangers on the floating roof and arranged vertically in sliding engagement with the sidewall of the tank, and a closure for the space between the roof and the sidewall of the tank constructed so as to provide for relative movement between the roof and the annular shoe while functioning as a primary seal.
Most liquid storage vessels having floating roofs presently use a mechanical seal. The mechanical seal consists of steel shoes pressed against the inner sidewall of a cylindrical tank by a hanger and pusher arrangement mechanically coupled to the floating roof and the steel shoes. Gas-tightness and durability are essentials of oil storage tanks and accordingly the principal requirements for the sealing arrangement of an oil storage tank of the described type are a uniform and a constant pressure between the inner sidewall of the tank and the floating roof, responsiveness to the movement of the floating roof relative to the inner sidewall of the tank, and resistance to wear and abrasion resulting primarily from the friction between the contact surfaces of the inner sidewall of the tank and the sealing structure. If the sealing pressure between the inner sidewall of the tank and the floating roof is deficient, gaps may be produced between the inner sidewall of the tank and the steel shoes although the sealing structure may be subject to lesser wear and abrasion that will result in a prolonged service life of the sealing structure. Oil vapor will therefore escape from the tank through such gaps, causing not only a considerable amount of loss of the stored oil but also the danger of a conflagration or explosion if a spark happens to be produced in the neighborhood of the leaks as in the event of an earthquake. If, conversely, the sealing pressure is excessive, improved sealing of the tank may be achieved and the relative movement between the floating roof and the tank may be accommodated to satisfactorily by the sealing arrangement but the wear and abrasion of the sealing structure will be accelerated resulting in a shortened service life of the sealing arrangement. Other important requirements for the sealing arrangement for the oil storage tank of the described type include: resistance to an undue stress which may be imparted to the sealing structure as a result of upward and downward movements or sways of the floating roof; configurations adapted to match the geometry of the tank and to be readily installed into a working position; and simple, economical and robust construction.